This invention relates to a keyboard apparatus, and more particularly to an improved capacitive-keyboard apparatus which detects the depression of a key from changes in electrostatic capacitance.
Keyboard apparatus have been used as input devices for inputting codes of characters, symbols, etc., to electronic computers. Each has a number of keys arranged substantially in one plane. Characters, numerals, symbols, signs, etc., are allotted to these keys. When any key is depressed, the apparatus outputs the code of the character, numeral, symbol or the like allotted to this key. An apparatus of this type has mechanical contacts.
Recently, a keyboard apparatus with capacitive switches has come into use. This apparatus is durable and has a good key touch. It includes keys connected to the capacitive switches, a sense amplifier, and a comparator with a specific hysteresis characteristic. Each switch has two electrodes. When any key is pushed, the interelectrode capacitance of the switch coupled to this key changes. The sense amplifier detects this change in the form of a voltage. It amplifies the voltage to a predetermined value. The amplified voltage is supplied to the comparator circuit and compared with a reference voltage. The comparator outputs the difference between the voltages as a key-depression signal.
It is demanded that the keyboard apparatus detect any key depression without fail. When one key is quickly depressed several times, the apparatus must generate the same number of key-depression signals even if the operator's finger touching the key shakes. To meet this demand, a new type of keyboard apparatus has been invented. In this apparatus, a comparator compares a voltage signal generated by depressing a key with its threshold voltage, and outputs a signal when the input signal reaches the threshold voltage. Then, the threshold voltage is lowered to a specified value. The comparator outputs a second signal upon lapse of a predetermined time from the threshold voltage change, and the apparatus then outputs a key-depression signal. However, when each capacitive switch is of the type that its interelectrode capacitance changes slowly as the corresponding key is depressed, the apparatus may output an erroneous key-depression signal.